Mixer circuits are commonly used to perform a frequency translation of a modulated input signal such that the output signal contains all the same phase and amplitude modulation present on the input signal translated onto a different output carrier frequency. The output carrier frequency can be either higher or lower than the frequency of the input carrier. These frequency translations are known respectively as upconversion and downconversion. Frequency translation of an input signal in this way necessarily results in degradation of the quality of the signal. Degradation is caused particularly by the addition of thermal noise, signal distortion through the use of non-linear components, the addition of signals harmonically related to both the frequency of the local oscillator used in the frequency translation and the input carrier frequency, leakage of the input signal to the output, and leakage of the local oscillator signal into both the input and the output. This latter effect is known as local oscillator “breakthrough”.
FIG. 1 shows a known balanced mixer circuit arrangement, known as a Gilbert cell mixer. The arrangement comprises generally an input stage 10, a mixer core 11, and a current source 12. The input stage comprises first and second npn transistors 13 and 14, each of which has its emitter electrodes connected to an output of the current source 12 by respective ones of emitter degeneration or series feedback resistors 15 and 16. The base electrodes of the first and second transistors 13,14 serve as the differential voltage inputs 13a,14a to the mixer circuit arrangement. The current source 12 includes a transistor 18, an emitter grounding resistor 19 and a capacitor 20, which is connected between the base electrode of the transistor 18 and ground potential. The base electrode of the transistor 18 is biased by a voltage source (not shown). The collector electrode of the transistor 18 forms the output of the current source 12. The current source 12 is a high impedance current source, which sets the quiescent current for the whole of the mixer circuit arrangement.
The mixer core 11 includes first to fourth npn transistors 21 to 24, the collector electrodes of which are cross-connected and connected to a supply line (Vcc) 25 by respective ones of load resistors 26 and 27. First and second output terminals 28 and 29 are connected to the lower terminals of the load resistors 26 and 27. Local oscillator input terminals 30 and 31 are connected to the base electrodes of the mixer core transistors 21 to 24.
Assuming that the Gilbert cell mixer is perfectly balanced, and in particular that the collector currents of the input stage transistors 13,14 are balanced, the output from the mixer core will consist mainly of the desired primary sidebands. However, any imbalance will tend to give rise to D.C. offsets in the circuit, one consequence of which will be local oscillator breakthrough into the output waveform of the mixer. Integrated Circuit (IC) layout techniques are optimised to achieve the best possible match, but this is limited by inherent mismatches found in IC devices, especially CMOS devices.